This invention relates to an improved packer for oil and gas wells. More specifically, the invention relates to an improved retrievable compression set straddle packer assembly to isolate a predetermined interval of perforations in an oil or gas well casing for the selective break-down of the perforations to remove debris and produced materials therefrom or the injection of stimulation fluids into the perforations to promote the free flow of formation fluids and gases into the well bore.
The perforated section of casing set across the producing zone in an oil or gas well can be many feet, sometimes several hundred, in length. To achieve the maximum stimulation effectiveness across the entire producing zone, as many perforations as possible must be open to accept stimulation fluids. Conventional practice has been to perforate the full length of the producing interval, then stimulate the entire zone in one operation. In long perforated intervals, some of the perforations will accept stimulation fluids more readily than others. These perforations then receive all the stimulation treatment leaving portions of the producing formation unstimulated.
To ensure that the maximum number of perforations within the perforated interval are open to accept stimulation fluids, short sections of the perforated interval may be isolated with a straddle packer of this type. Hydrochloric acid or other stimulation fluids can then be injected into the perforations isolated by the two sets of packing elements to break down the perforations in this short interval. This will insure that these perforations are open to produce reservoir fluids or to accept additional stimulation fluids. The shorter the perforated interval broken down at each packer setting the more perforations within the entire producing interval will be known to be capable of accepting other stimulation fluids or to produce reservoir fluids or gases.
Typically, such injection operations have been performed using a retrievable packer having frusto-conical type cups thereon to isolate the perforations to be washed. However, such cup-type packers are susceptible to damage of the frusto-conical cups as they pass over the perforations in the casing, pass through dry casing or pass through casing liners and cannot be used with high differential fluid pressures acting across the frusto-conical cups to prevent the cups from being damaged.
Therefore, in certain instances, it is desirable to have a retrievable compression set straddle packer assembly having two sets of compression set packer elements thereon to isolate perforations in the casing in the well bore for washing.
Also, it is desirable to have such a retrievable compression set straddle packer assembly to use as a selective injection packer to isolate portions of the well bore for fluid injection operations where the length of the packer assembly between the compression set packer elements may be easily varied for use in a wide variety of well operations.